Some lenses have very large or protruding front elements that make it impossible or impractical to use regular filters or filter systems. Instead, they may offer a slot towards the rear of the lens for inserting drop in filters, though the types available and what you can do with them are much more limited that regular filter systems.
Typical examples of lenses that use drop in filters are ultra wide-angle lenses with strongly convex front lens elements, and super-telephoto lenses with very wide front elements which would need impractically large filters or filter holders.